This invention relates to a low thermal expansion and high rigidity ceramic sintered body which is excellent in thermal stability and specific rigidity and usable in members of high precision control device, members of optical devices, and members demanding high thermal shock resistance which invariably abhor changes in size and changes in shape due to a thermal expansion or contraction induced by changes in temperature.
As materials which have been heretofore used under conditions demanding thermal stability, low thermal expansion metallic materials such as an Invar alloy (Fe-Ni type) and a super Invar alloy (Fe-Ni-Co type), low thermal expansion glasses such as quartz glass (SiO2) and quartz glass containing titanium oxide (SiO2-TiO2), and low thermal expansion ceramics such as aluminum titanate (TiO2.Al2O3), eucryptite (Li2O.Al2O3.2SiO2), xcex2-spundumene (Li2O.Al2O3.4SiO2), petalite (Li2O.Al2O3.8SiO2), and cordierite (2MgO.2Al2O3. 5SiO2) have been known. These materials are excellent in thermal stability because they have such a small thermal expansion coefficient as of not more than 1.2xc3x9710xe2x88x926/xc2x0C. in the neighborhood of a room temperature. Nevertheless they usually have a specific rigidity expressed by a ratio of a Young""s modulus to a specific gravity such values lower than 45 GPa/g/cm3 and, when used in members demanding dimensional stability and thermal shock resistance, therefore, are at a disadvantage in being readily deformed by an external force or under its own weight, offering only a low resonance frequency to the vibration of the relevant member, and generating a large amplitude.
The Invar alloy, for example, has a relatively small thermal expansion coefficient of about 1.2xc3x9710xe2x88x926/xc2x0C. at near a room temperature, a Young""s modulus of 144 GPa, a value rating high among low thermal expansion materials, a large specific gravity, and a small specific rigidity of 18 GPa/g/cm3. The super Invar alloy, though enjoying a small thermal expansion coefficient of 0.13xc3x9710xe2x88x926/xc2x0C., is deficient in mechanical stability because of a small specific rigidity of 17 GPa/g/cm3.
The quartz glass has a small thermal expansion coefficient of 0.48xc3x9710xe2x88x926/xc2x0C. and such an insufficient specific rigidity as of 33 GPa/g/cm3. The quartz glass containing titanium oxide has an extremely small thermal expansion coefficient of about 0.05xc3x9710xe2x88x926/xc2x0C. and is deficient in mechanical stability because of an insufficiently high specific rigidity of 33 Gpa/g/cm3.
Further, the aluminum titanate manifests negative expansion as evinced by a thermal expansion coefficient of xe2x88x920.8xc3x97106/xc2x0C. and has an extremely small specific rigidity of about 2 GPa/g/cm3. The lithium aluminosilicate type low thermal expansion ceramics such as eucriptite, xcex2-spodumene, and petalite have a small thermal expansion coefficient in the range of xe2x88x925 to 1xc3x9710xe2x88x926/xc2x0C., a not very high specific rigidity of about 35 GPa/g/cm3, and are deficient in mechanical stability. The compact sintered body of cordierite, though excelling various low thermal expansion materials mentioned above by exhibiting a specific rigidity of about 50 GPa/g/cm3, has a thermal expansion coefficient of 0.5xc3x97106/xc2x0C., which is not deserving to be called sufficiently low.
The invention described in JP-A-61-72,679, with a view to reducing a thermal expansion coefficient of cordierite capable of producing a relatively high specific rigidity, discloses a method which consists in attaining coexistence of a cordierite phase with a xcex2-spodumene phase as crystal phases and an auxiliary crystal phase as of spinel. This method has been reported to be capable of lowering a thermal expansion as compared with a simple phase of cordierete. With the same view as above, the invention described in JP-A-10-53,460 discloses a compact ceramics allowing the coexistence of a petalite phase, a spodumene phase, and a cordierite phase in a crystal phase. This compact ceramics has been demonstrated to excel in thermal shock resistance. Further, the invention described in JP-A-58-125,662 discloses a method for producing a ceramics allowing the coexistence of zircon in cordierite by adding a zirconium compound and a phosphorus compound to the cordierite. The sintered body obtained by this method has been reported to excel in thermal shock resistance. These materials, however, do not deserve to be rated as having a sufficiently low thermal expansion coefficient. As structural parts to be used for members in precision control devices, members in optical devices, and members demanding high thermal shock resistance, they cannot be said as having satisfactory thermal mechanical stability. Such has been the true state of the materials of interest.
Since conventional low thermal expansion ceramic materials are such that those having a small thermal expansion coefficient show a low specific rigidity and those having a high specific rigidity show no sufficiently low thermal expansion coefficient as described above, no low thermal expansion ceramic materials developed today have secured thermal stability such that the absolute value of thermal expansion coefficient does not exceed 0.1xc3x9710xe2x88x926/xc2x0C. while the specific rigidity is retained at a high level of not less than 45 Gpa/g/cm3, for example. Thus, conventional low thermal expansion ceramic materials have been at a disadvantage in being deficient in thermal reliability for members in precision structures.
It is, therefore, an object of this invention to provide a low thermal expansion ceramic sintered body which excels in thermal and mechanical stability and manifests both high specific rigidity and low thermal expansion coefficient.
The low thermal expansion and high rigidity ceramic sintered body of this invention is characterized by assuming as a crystal structure a hexagonal close-packed structure and substantially comprising solid solution crystal grains represented by the formula: MgaLibFecAldSieOf (wherein a is in the range of 1.8 to 1.9, b is in the range of 0.1 to 0.3, c is in the range of 0 to 0.2, d is in the range of 3.9 to 4.1, e is in the range of 6.0 to 7.0, and f is in the range of 19 to 23).
In the ceramic sintered body mentioned above, the solid solution crystal grains may preferably have lattice constants in such ranges, i.e., a0 in the range of 9.774 to 9.804 xc3x85 and c0 in the range of 9.286 to 9.330 xc3x85. The ceramic sintered body mentioned above may more preferably have a relative density of not less than 98%.
The present inventors, as a result of various studies, have found that in a sintered body formed solely of a single solid solution phase having as a crystal structure a hexagonal close-packed structure substantially represented by the formula: MgaLibFecAldSieOf excepting inevitable impurities, an absolute value of a thermal expansion coefficient can be controlled to not more than 0.1xc3x9710xe2x88x926/xc2x0C. and a specific rigidity to not less than 45 GPa/g/cm3 by controlling the ratios of each the component element within respectively prescribed range. When a second phase such as an amorphous phase having a large thermal expansion coefficient or a spinel phase assuming a cubic crystal structure is present in addition to the solid solution phase represented by the formula: MgaLibFecAldSieOf, the sintered body can not attain a sufficiently low thermal expansion coefficient. In order to obtain a sintered body having a high specific rigidity, it is desirable to incorporate a second phase such as an amorphous phase having a small specific rigidity, a xcex2-spodumene phase having a cubic crystal structure, or a xcex2-quartz solid solution phase in addition to the solid solution phase represented by the formula: MgaLibFecAldSieOf.
The solid solution crystal grains according to this invention may be substantially represented by the formula: MgaLibFecAldSieOf. In this formula, the variable xe2x80x9caxe2x80x9d may be properly in the range of 1.8 to 1.9. If the variable xe2x80x9caxe2x80x9d is smaller than 1.8, the shortage would be at a disadvantage in lowering a specific rigidity and tending to form a second phase such as a spodumene phase. If this variable xe2x80x9caxe2x80x9d is larger than 1.9, the excess would be at a disadvantage in increasing a thermal expansion coefficient exceeding 0.1xc3x9710xe2x88x926/xc2x0C. The variable xe2x80x9cbxe2x80x9d may be properly in the range of 0.1 to 0.3. If the variable xe2x80x9cbxe2x80x9d is smaller than 0.1, the shortage would increase a thermal expansion coefficient exceeding 0.1xc3x9710xe2x88x926/xc2x0C. If the variable xe2x80x9cbxe2x80x9d is larger than 0.3, the excess would decrease a specific rigidity. Further, the variable xe2x80x9ccxe2x80x9d in the formula may be properly in the range of 0 to 0.2. If this variable xe2x80x9ccxe2x80x9d exceeds 0.2, the excess would increase a thermal expansion coefficient and decrease a specific rigidity. If the variable xe2x80x9ccxe2x80x9d is smaller than 0.05, the shortage would cause the produced sintered body to assume a white color. When the variable xe2x80x9ccxe2x80x9d is in the range of 0.05 to 0.2, the produced sintered body would assume a gray color. The variable xe2x80x9cdxe2x80x9d in the formula may be properly in the range of 3.9 to 4.1. If the variable xe2x80x9cdxe2x80x9d is smaller than 3.9, the shortage would decrease a specific rigidity. If the variable xe2x80x9cdxe2x80x9d exceeds 4.1, the excess would possibly form an alumina phase as a second phase and markedly increase a thermal expansion coefficient. The variable xe2x80x9cexe2x80x9d in the formula may be properly in the range of 6.0 to 7.0. If the variable xe2x80x9cexe2x80x9d is smaller than 6.0, the shortage would possibly induce persistence of a xcexc-cordierite crystal phase in the sintered body and would increase a thermal expansion coefficient. Conversely, if the variable xe2x80x9cexe2x80x9d is larger than 7.0, the excess would markedly decrease a specific rigidity. The variable xe2x80x9cfxe2x80x9d in the formula may be properly in the range of 19 to 23. If the value xe2x80x9cfxe2x80x9d is smaller than 19, the shortage would increase a thermal expansion coefficient. If the value xe2x80x9cfxe2x80x9d is larger than 23, the excess would markedly lower a specific rigidity.
On the ground surface of the ceramic sintered body according to this invention, the lattice constants of the solid solution crystal grains analyzed by an X-ray diffraction method may be preferably within each the ranges, i.e., a0 in the range of 9.774 to 9.804 xc3x85 and c0 in the range of 9.286 xe2x88x929.330 xc3x85. If the lattice constant, a0 , is smaller than 9.286 xc3x85 or larger than 9.330 xc3x85, the shortage or excess would invariably prevent a thermal expansion coefficient from acquiring a sufficiently small absolute value of not more than 0.1xc3x9710xe2x88x926/xc2x0C. near a room temperature.
Further, the ceramic sintered body according to this invention may properly have a relative density of not less than 98%. If the relative density is less than 98%, the shortage would be at a disadvantage in markedly decreasing a specific rigidity.
The solid solution phase represented by the formula: MgaLibFecAldSieOf, according to this invention can be synthesized by forming in a required shape a mixed powder of compounds prepared in prescribed molar ratios, and sintering the formed body thereby causing the component compounds of the mixed powder to react in the course of sintering. Otherwise, the solid solution of MgaLibFecAldSieOf may be synthesized in advance by causing the mixed powder in a state neither formed nor sintered to be subjected to such steps as mixing, calcining, grounding, or electromelting. The raw materials which are usable herein may be known raw materials containing the elements of Mg, Li, Fe, Al, Si, O such as, for example, magnesium oxide (MgO) powder, magnesium hydroxide (Mg(OH)2) powder, lithium oxide (Li2O) powder, lithium carbonate (Li2CO3) powder, iron oxide (Fe2O3, Fe3O4) powder, aluminum oxide (Al2O3) powder, silicon oxide (SiO2) powder, spinel (MgAl2O4) powder, spodumene (LiAlSi2O6) powder, and petalite (LiAlSi4O10) powder. These raw materials may be arbitrarily combined so long as they form a solid solution having the variable xe2x80x9caxe2x80x9d to xe2x80x9cfxe2x80x9d of the formula: MgaLibFecAldSieOf representing the solid solution according to this invention in the respectively prescribed ranges.
The low thermal expansion and high rigidity ceramic sintered body obtained by this invention may assume as a crystal structure a hexagonal close-packed structure and comprise solid solution crystal grains represented substantially by the formula: MgaLibFecAldSieOf (wherein a is in the range of 1.8 to 1.9, b is in the range of 0.1 to 0.3, c is in the range of 0 to 0.2, d is in the range of 3.9 to 4.1, e is in the range of 6.0 to 7.0, and f is in the range of 19 to 23). The lattice constants of the solid solution crystal grains may preferably in such ranges, i.e., a0 in the range of 9.774 to 9.804 xc3x85 and c0 in the range of 9.286 to 9.330 xc3x85. More preferably, the ceramic sintered body mentioned above may have relative density of not less than 98%. As the result of the combination of these sets of conditions, the sintered body can manifest such an extremely low thermal expansion coefficient of not more than 0.1xc3x9710xe2x88x926/xc2x0C. as the absolute value at near a room temperature at which the relevant member of the sintered body may be used at a high frequency and at the same time, the specific rigidity represented by a ratio of Young""s modulus to specific gravity can be as high as not less than 45 GPa/g/cm3. Thus, the problems of this invention which resides in obtaining a sintered body excelling in thermal and mechanical stability by manifesting a low thermal expansion coefficient while maintaining a high specific rigidity can be solved.